The cardiomyopathies are an important, heterogeneous group of heart muscle diseases that make a significant contribution to morbidity and mortality. They are associated with mechanical and/or electrical dysfunction. Inappropriate ventricular hypertrophy or dilatation is usually present. Cardiomyopathy involvement may be predominantly limited to the heart (primary cardiomyopathy) or form part of a generalized systemic disorder (secondary cardiomyopathy). Causes vary widely, but genetic etiologies are most common in primary cardiomyopathies. Complications include cardiovascular death and progressive heart failure, with its associated disability.
In 1995, the World Health Organization/International Society and Federation of Cardiology Task Force classified cardiomyopathies as primary myocardial disorders, whereas heart muscle diseases of known etiology or associated with systemic diseases were categorized as secondary or specific heart muscle diseases. However, as research has improved the understanding of these conditions, working groups on both sides of the Atlantic have proposed new but different classification systems.
The scientific statement from the American Heart Association has defined cardiomyopathies as "a heterogeneous group of diseases of the myocardium associated with mechanical and/or electrical dysfunction that usually (but not invariably) exhibit inappropriate ventricular hypertrophy or dilatation and are due to a variety of causes that frequently are genetic. Cardiomyopathies either are confined to the heart or are part of generalized systemic disorders, often leading to cardiovascular death or progressive heart failure-related disability."
Primary cardiomyopathies are those where the condition is predominantly confined to the heart muscle and where subclassifications of genetic, mixed, and acquired are adopted.
Secondary cardiomyopathies are those where myocardial involvement occurs as part of a systemic or multiorgan disorder.
The European Society of Cardiology Working Group on Myocardial and Pericardial Diseases has opted to use a clinical rather than genetic classification, where heart muscle disorders are classified according to morphology and function: "a myocardial disorder in which the heart muscle is structurally and functionally abnormal, in the absence of coronary artery disease, hypertension, valvular disease and congenital heart disease sufficient to cause the observed myocardial abnormality." Its members felt that many primary cardiomyopathies have significant extracardiac manifestations and that many secondary cardiomyopathies may involve the heart as the major manifestation.
It is important to recognize that the traditional classification into hypertrophic, dilated, and restrictive cardiomyopathies mixes anatomical with functional designations, which are not mutually exclusive.
The American Heart Association published a separate scientific statement in 2019, covering the classification and diagnosis of cardiomyopathy in children.  This favours a classification for paediatric cases based primarily on the structural and functional phenotype (which forms the basis for diagnosis and management), with genetic and nongenetic causes as lower-level subcategories.
- Idiopathic dilated cardiomyopathy
- Alcohol: dilated cardiomyopathy
- Hypertrophic cardiomyopathy (HCM)
- Idiopathic restrictive cardiomyopathy
- Arrhythmogenic right ventricular cardiomyopathy/dysplasia (ARVC/D)
- Left ventricular noncompaction
- Brugada syndrome and other ion channelopathies
- Conduction system disease
- Mitochondrial disorders
- Stress provoked (tako-tsubo)
- Peripartum: dilated cardiomyopathy
- Tachycardia induced: dilated cardiomyopathy
- Amyloidosis: hypertrophic or restrictive cardiomyopathy
- Hemochromatosis: restrictive or dilated cardiomyopathy
- Fabry disease: hypertrophic or restrictive cardiomyopathy
- Other lysosomal storage disease
- Doxorubicin: dilated cardiomyopathy
- Heavy metals/chemicals: dilated cardiomyopathy
- Diabetes mellitus
- Thyroid dysfunction: dilated cardiomyopathy
- Acromegaly: hypertrophic or dilated cardiomyopathy
- Noonan syndrome: hypertrophic cardiomyopathy
- Lentiginosis: hypertrophic cardiomyopathy
- Thiamine deficiency (wet beriberi): dilated cardiomyopathy
- Friedreich ataxia/muscular dystrophy: hypertrophic and dilated cardiomyopathy
- Deficiency of iron, niacin, selenium, or vitamin D: dilated cardiomyopathy
- Systemic lupus erythematosus: dilated cardiomyopathy
- Endomyocardial fibrosis/Loeffler endocarditis (hypereosinophilic syndrome)
- Electrolyte disorders
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